1. Field of the Invention
The present invention relates to switching power supply devices and, in particular, relates to a resonance type switching power supply device that utilizes a resonance phenomenon for an electric power conversion operation.
2. Description of the Related Art
A switching power supply device described in Japanese Unexamined Patent Application Publication No. 2008-228382 turns on/off a high side switching element and a low side switching element in an alternating manner so as to generate a resonance voltage in a primary winding of an insulated transformer, on the basis of a voltage generated in two auxiliary windings provided in the insulated transformer. As a result, the switching power supply device outputs a constant direct current voltage through a secondary winding of the insulated transformer. The switching power supply device described in Japanese Unexamined Patent Application Publication No. 2008-228382 is configured such that the primary winding is wound between the two auxiliary windings and the secondary winding. Through this, the degree of coupling between the two auxiliary windings and the primary winding is increased, and the degree of coupling between the two auxiliary windings and the secondary winding is reduced. This makes a voltage generated in the two auxiliary windings similar to a voltage generated in the primary winding, and a ZVS (zero voltage switching) operation is thus achieved.
In the switching power supply device described in Japanese Unexamined Patent Application Publication No. 2008-228382, it is desirable that the low side switching element operate with a waveform that is similar to the waveform of a voltage generated in an inductance of the primary winding and that the high side switching element operate with a waveform that is similar to the waveform of a voltage generated in an inductance of the secondary winding. Then, an operation such that the high side switching element and the low side switching element are turned on/off in an alternating manner is achieved more reliably. In other words, a low side drive winding needs to enhance its coupling with the primary winding, and the voltage waveforms of the low side drive winding and of the primary winding need to be made similar to each other. Meanwhile, a high side drive winding needs to enhance its coupling with the secondary winding, and the voltage waveforms of the high side drive winding and of the secondary winding need to be made similar to each other.
However, in the switching power supply device described in Japanese Unexamined Patent Application Publication No. 2008-228382, the primary winding is present between the drive windings (the high side drive winding and the low side drive winding) and the secondary winding, and thus the degree of coupling between the secondary winding and the auxiliary windings cannot be increased. In addition, in Japanese Unexamined Patent Application Publication No. 2008-228382, the physical distance between the primary winding and the secondary winding is small, and a leakage inductance generated through coupling between the primary winding and the secondary winding is reduced. Therefore, desired excitation inductance and leakage inductance cannot be generated. Furthermore, a distance in accordance with a safety standard needs to be provided between windings, which restricts design flexibility.